can I use automotive body filler over the primer/topcoat, then sand and paint ?
Not really, You don't walk on a car body. Layers of primer sanded should help but perhaps a coating of nonskid may reduce the appearance of them. The body filler won't wear and will break loose exposing the primer or metal. It will end up looking worse.
As far as POR-15 or RustBullet these really are not tried and true marine
products. You may want to seek other advice. When you combine metal with salt water
that is as nasty an enviroment as possible. The products you are considering maybe better than Rustoleum but the bar needs to be a whole lot higher for a boat. A metal deck
even more difficult if not the most difficult. This isn't a job you want to repeat often.
Here is a product I was considering but never pursued for a fiberglass deck
but it is considered an industruial type product for a metal deck as well. It may not be exactly what you want but it could give you some better ideas. This is a marine
grade product. Preparation for all products is the most important step.
Ultra TUFF/TUFFcoat Anti-Slip, Non-Slip, Non-Skid safety coatings manufacturer.